1. Field of the Invention
The present invention generally relates to processes, apparatus, and articles relating to presswork, stamping, and cold forming of metal. More specifically, this invention relates to deep drawing of metal plate into a drawn part for an automotive suspension system.
2. Description of the Related Art
Successful drawing is a process of forming a flat blank of metal, or workpiece, to convert the blank into a hollow shell without unacceptable defects such as cold-shutting, wrinkling, puckering, tearing, thinning, or fracture. The process involves placing the blank on top of a lower die section over an opening die cavity thereof and using an upper die punch to force the blank into the die cavity under forces sufficient to draw the blank past an edge of the opening and into the die cavity, so as to form the hollow shell. Additionally, a ring-like blank holder is usually employed to hold the periphery of the blank in place. The blank is thereby confined between the blank holder and the top of the lower die portion, with the amount of pressure exerted on the blank being sufficient to prevent the part from incurring the above-mentioned defects. Where heavier gage blanks are drawn, however, the thickness of the metal may be sufficiently internally resistant to wrinkling and, therefore, may not require a hold-down device.
Typical metals used for drawing include carbon and alloy steel, and aluminum. Parts that may be drawn include those having simple cylindrical shapes, or more complex geometries having square or rectangular cross sections. For cylindrical shapes, when the depth of the drawn part exceeds the diameter of the part, the process is generally considered to be deep drawing and usually requires multiple progressive drawing operations; otherwise, the process is generally referred to as shallow drawing and can be drawn in one press operation. For rectangular shapes, it is generally known that such parts can be successfully drawn to a maximum depth equal to approximately six times the size of the corner radius on the part without incurring the above-mentioned defects.
Parts that cannot be successfully drawn in one step must, if at all possible, be redrawn in subsequent progressive steps, and often require an annealing operation between such steps to restore the ductility of the metal for further cold working thereof. Such parts tend to be heavy gage sheet metal greater than 0.100xe2x80x3 in thickness. Heavier gage parts exceeding 0.250xe2x80x3 are characterized as plate steel rather than sheet steel and attempts to successfully deep draw such thicknesses have been limited due to the resistance of such heavy gage materials against entering the draw section of the lower die portion. Defects are often experienced, such as thinning of the wall sections, punching through the bottom of the part, or cracks and splits at the edges or radii of the part. Often times, wrinkling, or cold-shutting, occurs wherein the metal folds over upon itself. Much worse, however, such xe2x80x9cexperimentsxe2x80x9d with heavy gage steel often lead to catastrophic damage to the extremely expensive tools and stamping press equipment involved. Therefore, many manufacturers are understandably reluctant to attempt single stroke, single operation deep drawing on such heavy gage plate steel.
Instead, manufacturers typically avoid drawing such parts altogether and use expensive superplastic forming or hydro-forming techniques, or attempt to draw the parts in multi-step progressive drawing operations using stamping presses with multiple stations. For example, U.S. Pat. No. 4,147,049, to Book et al. teaches a method and machine for drawing heavy-walled parts. Book et al. disclose that the method is carried out on a multiple plunger machine including no less than six separate press stations. Book et al. further disclose that each press station uses a supplemental sleeve that circumscribes a die punch. The sleeve assists the die punch in drawing a cup into a die cavity by contacting an annular edge at an open end of the cup to reduce the tensile stress in the cylindrical portion of the cup.
In another example, U.S. Pat. No. 4,509,356, to Budrean et al. also teaches a method and apparatus for drawing heavy walled shells and points out shortcomings in the teachings of Book et al. Specifically, Budrean et al. assert that the annular edge at the open end of the cup does not always remain perfectly square with the axis of the cup, so that the sleeve cannot uniformly reduce the tensile stress in the cup. Instead, Budrean et al. disclose a method for use on a forming machine that includes no less than seven separate press stations. Each station uses a die punch having a reduced diameter for engaging an inside diameter and bottom portion of the cup. Each die punch also includes an enlarged diameter that defines an annular step. At the first station, the annular step of the die punch forms a complementary annular step at the upper inside diameter of the cup that remains square with the axis of the cup. At subsequent stations, the steps in the subsequent die punches locate on the step in the cup to uniformly reduce tensile stress in the cylindrical portion of the cup.
Unfortunately, both Book et al. and Budrean et al. disclose extremely expensive die arrangements involving multiple stations. Additionally, such processes involve significantly longer process time than a single station process, and necessarily involve more failure modes and downtime than a single station process.
Turning now to a more specific problem in the prior art, forming of leaf spring seats for automotive suspension systems has not been, heretofore, conducive to deep drawing. In particular, 4xc3x974 trucks usually incorporate relatively tall spring seat and spacer block assemblies to provide the appropriate height between the body and the chassis. Such spring seats are generally made from xe2x85x9cxe2x80x3 thick plate and, therefore, have been impractical to deep draw effectively in one operation to the required height of approximately 3xc2xdxe2x80x3. As is well known in the art, such a part is not possible to produce in a one-step operation, due to the aforementioned defects that are inherent with a one-step stamping operation, on such a deeply drawn heavy gage part made of steel. For example, it is recognized by those skilled in the art that the maximum overall depth that can be successfully drawn on a rectangular shell having a radius of approximately xc2xcxe2x80x3, as with the present invention, is only approximately 1xc2xdxe2x80x3.
Before the present invention, attempts to exceed these drawing guidelines typically resulted in thinning at the corners, tearing out in the bottom of the stamped cavity, and wrinkling or cold-shutting at the ends of the leaf spring seat. Therefore, such a part can typically be produced in multiple steps using a progressive die process, as exemplified above by Book et al. and Budrean et al. More typically however, a much shorter leaf spring seat is used in conjunction with a spacer block to achieve the desired height of the parts, as is very well known in the art and exemplified by U.S. Pat. No. 2,678,819 to Douglass. For example, as taught by Douglass, such prior art solutions typically use a relatively short spring seat 6 having a height of approximately 2xe2x80x3 in combination with a 1xe2x80x3 or taller riser block 8.
Any of the above-mentioned solutions as taught by the prior art are relatively expensive and complex. A multiple-operation progressive die process involves very expensive machinery and tooling, excessive process time, and work-in-process between stations of the stamping press. Similarly, use of a leaf spring bracket and spacer block assembly involves a less robust design, assembly that is more complicated, and a higher overall cost and weight. The spacer block solution is less robust since the spring seat and spacer block inevitably wear against one another, inclusion of extraneous parts tends to increase failure modes within a system, and less precise location of the spring to the spring seat will be achieved. Finally, assembly of the suspension system is needlessly complicated because it entails assembling the extra spacer block.
What is needed, therefore, is a method of deep drawing a heavy gage part that may be accomplished with a single action die and continuous single stroke operation, and does not require a progressive die operation, annealing steps, or use of a blank holder device. Additionally, a spring seat that is deep drawn in a single operation is needed to replace spring seat and spacer block combinations of prior art suspension systems.
According to the present invention, there is provided an apparatus and a method of stamping or deep drawing a heavy gage part that may be accomplished using a single action die with continuous single-stroke press operation, and does not require a progressive die operation, annealing steps, or use of a blank holder device.
In one form of the invention, an apparatus is provided for drawing a heavy gage workpiece into a drawn part. The apparatus includes a lower die section having a die cavity therein and a base mounting surface thereon. The lower die section typically includes a die radius defined between the base mounting surface and the die cavity to assist the entry of the workpiece into the die cavity. Riser blocks are mounted to the base mounting surface of the lower die section for supporting the workpiece at a predetermined elevation or spacing from the base mounting surface of the lower die section. Accordingly, the workpiece is located on top of the riser blocks instead of the base mounting surface. An upper die punch reciprocates past the riser block and into and out of the die cavity. The upper die punch includes an end face thereon that contacts the workpiece as the upper die punch is advanced from a retracted position to a lowered position into the die cavity. The upper die punch and the riser blocks cooperate to pre-draw the workpiece therebetween as the upper die punch moves toward the lower die section, but before the workpiece enters the die cavity of the lower die section. Thus, the upper die punch and riser blocks cooperate to act upon the workpiece to prematurely start metal working in areas of the workpiece that are typically prone to defects.
In another form of the invention, a method of drawing a heavy gage workpiece into a drawn part is provided that includes the following steps. First, a lower die section is provided with a base mounting surface and a die cavity therein, with the lower die section further having a die radius defined between the base mounting surface and the die cavity. Second, at least one riser block is provided and is mounted to the base mounting surface of the lower die section for supporting the workpiece at a predetermined spaced apart distance from the base mounting surface and die cavity of the lower die section. Third, an upper die punch having an end face that reciprocates past the riser block and into and out of the die cavity is provided. Beneath the upper die punch, the workpiece is located on the at least one riser block just above the die cavity. Finally, the upper die punch is advanced from a retracted position to a lowered position into the die cavity, wherein the end face of the upper die punch first contacts the workpiece located on the at least one riser block, such that the upper die punch and the riser block cooperate to pre-draw the workpiece therebetween as the upper die punch moves toward the die cavity, but before the workpiece and upper die punch enter the die cavity of the lower die section.
Specifically, the riser blocks support one surface of the workpiece in areas where defects typically originate. For example, the risers support an area proximate the corners of the workpiece that correspond to end portions of the final part that would otherwise incur cold-shutting defects. Conversely, the upper die punch drives an opposite surface of the workpiece at a central portion thereof, so as to contact, and bend or flow the workpiece between the upper die punch and riser blocks. Thus, the upper die punch and riser block cooperate upon the workpiece to start metal working or flowing, in areas of the workpiece that are typically prone to defects in order to prevent such defects from occurring as the single stroke punch continues to push the metal workpiece into the die cavity so as to progressively conform the bottom of the workpiece to the bottom of the die cavity and simultaneously form each of the corners and straighten out sidewalls to final form. By beginning to form corners of the workpiece before it enters the cavity, heat is generated by the metal forming and assists in forming the bottom and walls of the workpiece without wrinkles or other aforementioned defects.
In a final form of the invention, an article, such as a spring seat, is produced by the method of the present invention as described above.
Accordingly, it is an object of the present invention to provide an apparatus and method to pre-form a blank of material before the blank enters or contacts a die cavity by locating at least one riser around the periphery of the die cavity, such that the blank is introduced to the die cavity at an oblique or acute angle of incidence rather than from a horizontal plane.
It is another object to provide an apparatus and method for deep drawing a heavy gage blank in a single station press, single action operation without the blank incurring significant drawing defects.
It is yet another object to provide an apparatus and method for forming, in a single hit, a drawn part from a steel workpiece whose depth significantly exceeds six times the size of the corner radii of the part.
It is still another object to provide an apparatus and method for deep-drawing a heavy gage part that is less expensive and that requires less process time than conventional progressive die forming of such parts.
It is a further object to provide an apparatus and method for deep-drawing a heavy gage part that requires only a single station rather than several progressive stations, and therefore has less failure modes and downtime.
It is yet a further object to replace a multiple piece spring seat and spacer block combination with a one piece spring seat of equivalent effective height.
It is still a further object to provide a lower die section having upright extensions extending therefrom for locating a workpiece thereon and that contributes to accomplishing the above-listed objects.
It is another object to provide an apparatus and method for stamping a heavy gage part on a single station press where pre-forming of the workpiece occurs before the workpiece enters the die cavity so that heat generated by such pre-forming assists in the formation of a deep drawn heavy gage metal workpiece.
It is another object to provide an apparatus and method for stamping a deep drawn part wherein work performed on the workpiece before the workpiece enters the die cavity generates heat so as to begin flow of the metal in the areas of the blank that are subject to unacceptable defects.
It is another object to provide an apparatus and method for stamping a deep drawn part wherein heat generated in the workpiece prior to the punch entering the cavity of the die is utilized in subsequent forming of the workpiece to assist in flowing of the metal within the cavity of the die so as to assist in the formation of the bottom and sides of the workpiece of a deep drawn workpiece.
These objects and other features, aspects, and advantages of this invention will be more apparent after a reading of the following detailed description, appended claims, and accompanying drawings.